Refractory articles bonded to a metal base



Dec. 8, 1964 R. A. ALLIEGRO 3,160,480

REFRACTORY ARTICLES BONDED TO A METAL BASE Filed May 11 1961 INVENTOR RICHARD A. ALLI EGRO ATTORNEY United States Patent 3,160,484) REFPACTQRY ARTICLES BGNDEB TQA METAL BASE Richard A. Alliegro, fiolden, Mass, assignor to Norton fiompany, Worcester, Mass, a. corporation of Massachusetts Filed-May 11, 1961-, Ser. No. 109,452 3 Claims. (ill. 29l) This invention relates to the bonding of refractory articles to supoprting metallic bases and to the composite and integral articles thereby produced,. More particularly, this invention relates, to a method of bonding refractory articles formed from borides or carbides to bases of iron, steel, or other conductive metals to provide composite articles wherein the desirable characteristics of the refractory material may be conveniently utilized.

The application of refractories such as borides and carbides in various industrial processes is becoming more and more prevalent. Such materialsas titanium diboride, zirconium diboride and titanium carbide, for example, when used as cathodes in electrolytic cells, have shown distinct advantages over the standard iron cathode conventionally utilized. One specific application is the use of these materials in cells for they reductionof alumina to aluminum metal.

Titaniumdiboride, for. example, when fabricated to certain specifications, ischaracterized by a high density (above 88 percent of its theoretical value), low electrical.

resistivity (less than 30 micro-ohm centimeters), high strength (greater than 40,000 p.s.i. cross bending at room temperature), low solubility in molten aluminum and excellent resistance to the electrolytes such as cryolite used in aluminum cells. Zirconium diboride and titanium carbide exhibit similar properties.

One major problem associated with the use of. these materials is the cost of the materials themselves both in powder form and in fabricated shapes. The. production of borides and carbides of high purity is expensive and fabrication into shapes necessitates the use of great degrees of heat and pressure because of the relative refractoriness of these materials. Cathodes for electrolytic cells are produced in the-form of bars and range in size from. about 2. to 8 inches in diameter and from about 18 to 24 inches in length while the Working end or tippertion: actually in contact with the aluminum is only about 2 to, 4 inches of this length. The remainder of the bar extends from the working endand is connected to a source of'electrical power.

It has been found that a considerable saving of expense in the production of cathodes may be achieved if only the tip or that portion. in or adjacent the electrolyte bath is formed ofthe costly refractory and the remainder formed of a cheaper metal such as iron or steel. However, the direct bonding of metal to the tip such as by Welding has been found to be impractical largely due to the. great difierence. in coefficients of expansion of these materials. Titanium diboride, for example, has a coefficient of expansion of 8.1x 10- over the temperature range of'20 to 800 C., While iron or steel-has an average coefiicient of expansion of about 14x 10' over the temperature range of 20 to. 700 C. Furthermore, the refractories-suchas titanium diboride lack ductility. Therefore, the expansion and. contraction of a directly welded bar upon heating and cooling would produce undesirable stresses and the danger of brittle fracture of the intermetallic material together with disintegration of the weld interface. Obviation' of these stresses is important since tobe effective as an electrode, the composite bar must remain integral to insure a low electrical resistivity over a wide temperature range.

It is. therefore an object of this invention to provide Still further objects and the entire scope of applicability p of the present invention will become apparent from the detailed description given hereinafter; it should be-unden stood, however, that the detailed description, while indicating preferred embodiments of the invention, is given by way. of illustration only, since various changes and modifications withinthe spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

It has been found that the above objects may be attained by bonding the refractoryv article to a less expensive metallic base or shank by a novel-method in which an alloy having desirable electrical resistivity and a co-- efficient of expansion compatible with that of the refractory material is utilizedinforming the bond.

More particularly, the alloy is interposed between the refractory article and the metallic base and themembers are bonded together with a special solder adapted to Wet the refractory article. The bond is effected byheating the assembled article in a protective atmosphere to a temperature above the flow point of the-solder'but below the melting point of the members to be joined; The bonded assembly is. then slowly cooled, the; initial portion: of the cooling cycle preferably conducted also in the protective atmosphere. p

7 With reference to an embodiment ofthis invention relating to the production of a cathode, a refractory article, for example, in the form ofarod isinserted into an alloy cup into which has been placed the special solder. Beneath the alloy cup is placed the base of a metal of desired electrical resistivity, a portion of the" same solder being used between the alloy cup andthe metallic base. The assembled bar is placed in afur nace in a protective or an inert atmosphere, e.g., an atmosphere of an inert gas such as nitrogen, argon, hydrogen or helium, and heated to atemperature in excess of 1000 C. and preferably about-1100 0 This temperature is held for a time elfective to insure complete flow of the solder, e.g., for about 10 minutes. The composite bar is then allowed to cool. very slowly to insure against thermal breakage, 24 hours usually'being sufiicient. The protective atmosphere. is maintained during the heating. and soaking periods but it may be eliminated after the temperature drops below about 1000 in the cooling cycle.

The complete manufacture of suchan electrode. may be considered as broken down into two steps: first, that of providing the refractory tip which will project into the molten aluminum and second, that of joiningthe tip to a metallic base as outlined above. The refractory tip may be produced by standard powder metallurgy techniques including cold pressing and sintering or hot press ing. The tips mentioned in the detailed description below were prepared. by hot pressing although articles produced by other techniques may be likewise treated.

An alloy which has been found to beparticularly suitable for the purposes of this invention is a nickel-ironcobalt alloy metal known commercially as Kovar by which name it. will be referred to herein. The composition of this alloy is 29% nickel, 17% cobalt, about 012% manganese and the remainder iron. There are a,

number of. other alloys which might be employed in this nvention but Kovar has been found to be especially iuitable. The requirements placed upon the alloy are Jhat it have an electrical resistivity in the general range at those of the intermetallic and metal utilized, that it lave a coefiicient of expansion compatible with that of :he intermetallic and that it can be subjected to high tem- Jeratures. As set forth above, titanium diboride has .a :oefiicient of expansionof 8.1 10- over the tempera- :ure range of 20 to 800 C., while iron or steel has in average value of about 14X 10' over the temperature 4 component parts and forming a strong bond therebetween.

A bar as formed in the above embodiment was tested in an experimental electrolytic cell built to simulate an actual aluminum production cell. Cycling between room temperature and 1000 C. did not disrupt the bond. Af-

1000 C. and 12 days in molten cryolite at 1050 C., the

0 average resistivity for the bar was the same as when the ange of 20 to 700 C. Kovar alloy has an average'coeffi- :ient of expansion of 8.0 l0 over the temperature range of 20 to 700 C., almost identical to that of titests were first started. This indicates the high integrity of the bond. Also, the low average resistivity of 22 micro-ohm centimeters for the bar at room temperature about doubled at 1000" C. to 45 micro-ohm centimeters while carrying 100 amperes of current. 1

- A second embodiment of this invention is illustrated in FIGURES and is especially useful when bars of comfor titanium diboride and iron or steel being 15-25 md 10-20 micro-ohm centimeters respectively, also at 00m temperature. The function of the Kovar in the :omposite electrode is to prevent separation of the bond while providing a good path for the flow of current. The solder which is utilized must allow for a low :lectrical resistivity, a high flow temperature, and provide in effective bond. One such solder contains a major roportion of silver and a minor proportion of mangaiese. The silver constituent provides the desired electrizal properties and the manganese is important in that it vill wet the refractory, whilesilver by itself will not. an alloy containing 85% silver and 15% manganese has I. flow temperature of 950 C. and combines the wetting tction of manganese with the desirable electrical properies of silver to produce an ideal bond.

Referring now to the drawing wherein like figures re- 'er to like parts,

FIGURE 1 is a cross-sectional view of an assembly at a refractory article and a metallic base prepared ac- :ording to one embodiment of this invention;

FIGURE 2 is a cross-sectional View of the assembly f FIGURE 1 after treatment according to this invenvf iron or steel. Within the Kovar cup and also between he bottom of the cup and the base are placed thin shims l of silver alloy, several sheets beingplaced withinthe :up so that during flow of the molten solder, the void is :ompletely filled. Prior to assembly, the parts to be ronded, namely the tip, the Kovar 'interfacial material tl'ld the iron or steel 'base are sand or shot blasted to oughen the surface and to remove any foreign matter, vnd further cleaned with organic solvents such as toluol, methylene chloride or acetone. The shims of silver solder re in this embodiment cut into circles, the diameters of vhich match the parts to be joined, and are also cleaned vith the solvent.

The bar assembled as in FIGURE 1 is placed in a 'urnace capable of being heated to a temperature of 200 C. in an hour and containing a protective or inert .tmosphere. The firing temperature used in this embodiment was 1100 C., but this may vary depending upon he type of solder utilized and the density of the refracory material used in the tip. The bar was held at this emperature for 10 minutes insuring a good flow of the ilver solder. The bar was then cooled over a time period if 24 hours, the inert atmosphere being unnecessary after he temperature fell below 1000 C. The completed comlosite and integral bar as shown in FIGURE 2 comprises he tip 1, Kovar cup 2'and base 3 bonded by the silver older 5, now completely filling the voids between the aluminum metal from alumina.

' paratively large diameter are produced. In this embodiment, in place of the Kovar cup 2, shown in FIGURES l and 2,a simple plate 6 of the Kovar alloy may be utilized. The base '7 is modified to provide a generally cupshaped end portion 8. The Kovar plate 6 is placed within cup portion 8 already having silver solder in place at the bottom thereof. Additional solder is placed upon the Kovar plate. The end portion of refractory tip 9 which is inserted into cup portion 3 of base 7 is beveled slightly at 10 to correspond to the inner surface 11 of cup portion 8.. After heating and cooling as outlined above, the solder has melted and flowed to fill the spaces 5 between the elements to form aninte'gral bond therebetween.

The bars formed according to the preferred embodiments of this invention fulfill the requirements of cathodes'to be used in electrolytic cells for the production of resistivity of the bars is between 20 and '30 micro-ohm centimeters at room temperature. The bond formed is refractory, withstanding temperatures in excess of 800 C.,' and a bar when cycled from, room temperature to 1000 C. in the cell maintains its integrity and therefore its low electrical resistivity. Moreover, this invention provides the additional advantage of a saving up, to twothirds of the cost of the intermetallic material normally needed in the production of a cathode.

What is claimed is: l. A composite bar suitable for use as an electrode in an electrolytic cell comprising:

(1) a first member having low solubility in molten alumina said member being formed entirely from a material-selected from the group consisting of titanium boride, titanium carbide,'and zirconium boride; (2) .a metallic member selected from the group consisting of iron and steel and having a coefiicient of expansion greater than that of said first member;

(3) a ferrous alloy. member having a coeflicient of ex:

pansion substantially equal to that of said first member and less than that of said metallic member interposed between said first member and said metallic member; said members being bonded by a silver-manganese solder capable ofwetting said first member. 2. The bar according to claim 1 wherein the solder comprises about silver and about 15% manganese.

3. The bar according to claim l'wherein the first member is titanium diboride.

References Cited in the file of this patent UNITED STATES PATENTS 1,524,218 Smith Jan. 27, 1925 2,141,113' Peterson Dec. 20, 1938 2,439,570 Hensel Apr. 13, 1948 2,708,787 Chick Nay 24-, 1955 "FOREIGN PATENTS 671,072 Great Britain Apr. 30, 1932 The overall electrical 

1. A COMPOSITE BAR SUITABLE FOR USE AS AN ELECTRODE IN AN ELECTROLYTIC CELL COMPRISING: (1) A FIRST MEMBER HAVING LOW SOLUBILITY IN MOLTEN ALUMINA SAID MEMBER BEING FORMED ENTIRELY FROM A MATERIAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM BORIDE, TITANIUM CARBIDE, AND ZIRCONIUM BORIDE; (2) A METALLIC MEMBER SELECTED FROM THE GROUP CONSISTING OF IRON AND STEEL AND HAVING A COEFFICIENT OF EXPANSION GREATER THAN THAT OF SAID FIRST MEMBER; (3) A FERROUS ALLOY MEMBER HAVING A COEFFICIENT OF EXPANSION SUBSTANTIALLY EQUAL TO THAT OF SAID FIRST MEMBER AND LESS THAN THAT OF SAID METALLIC MEMBER INTERPOSED BETWEEN SAID FIRST MEMBER AND SAID METALLIC MEMBER; SAID MEMBERS BEING BONDED BY A SILVER-MANGANESE SOLDER CAPABLE OF WETTING SAID FIRST MEMBER. 